The polyolefins, and particularly polypropylene (used herein to include propylene homopolymer and copolymers of propylene with up to about 15% of a second alpha-olefin of 2 to 4 carbon atoms), have a number of properties which make them highly desirable as packaging materials. For example, they are inexpensive and tough, have good surface properties, are readily drawn abnd shaped, are inert to most chemical materials and can be employed in food packaging applications. Polypropylene is, however, subject to the very serious objection that it is highly permeable to oxygen and other gases which can be harmful to food products. To overcome this objection for many applications, the polypropylene is laminated with another polymer which has better gas permeability characteristics.
Chlorinated hydrocarbon polymers such as polyvinyl chloride, poly(vinylidene chloride), and copolymers of the preceeding, have been found to be excellent gas barrier materials. It has been proposed to employ a layer of one of these materials as an interlayer between layers of the polypropylene to enhance the gas barrier properties of the composite structure. A layer of a film-forming saponified ethylene--vinyl acetate copolymer has also been found to be useful for this purpose.
Due to significant differences in polarity between polypropylene and the materials which function most satisfactorily as gas barriers, polypropylene does not adhere well to the barrier film, so it is necessary to employ an adhesive layer between the polypropylene and the barrier layer in order to preserve the desired laminar structure. A number of materials have been suggested for this purpose. The most popular appears to be an ethylene--vinyl acetate copolymer.
It is the object of this invention to provide alternative materials which function as well or better than the materials presently known for use as the aforesaid adhesive layer.
It is a further object to provide a modified propylene polymer which exhibits excellent adhesion to both polypropylene and chlorinated hydrocarbon polymers.
It is another object to provide laminar polypropylene structures exhibiting favorable gas permeability characteristics for use as food containers and the like.